He probably closed the shutter while he lit a cigarette. Seriously, you often see these gaps in time exposures when the astrophotographer needs to save the picture from temporary light pollution, like a vehicle driving up, or the brief use of a flashlight while adjusting his equipment or consulting a chart.

That gap appears in every star trail, so by overlaying a clear acetate over the image and marking each gap, you would get a star map of the circumpolar regions with recognizable constellation patterns.

For example, if a meteor flashed across the sky during his time exposure, by interrupting it he could get a fairly accurate plot of its path through through the star field.